1. Field of the Invention
Aspects of the present invention relate to a phosphorous containing benzoxazine-based monomer, a polymer thereof, an electrode for a fuel cell including the same, an electrolyte membrane for a fuel cell including the same, and a fuel cell employing the same.
2. Description of the Related Art
Fuel cells using a polymer electrolyte membrane as an electrolyte, which operate at a relatively low temperature and can be miniaturized, are regarded as an alternative power source for automobiles and for residential distributed power generation systems. A known polymer electrolyte membrane used in polymer electrolyte membrane fuel cells is a perfluorocarbonsulfonic acid polymer represented by Nafion™.
However, these polymer electrolyte membranes typically must be hydrated to retain proton conductivity. In addition, a fuel cell system typically operates at 100° C. or higher in order to improve the system efficiency. However, the electrolyte membrane may not function well as a solid electrolyte at such a high temperature since moisture evaporates from the electrolyte membrane.
A non-hydrated electrolyte membrane that can be operated at 100° C. or higher has been developed in order to overcome these problems. For example, polybenzimidazole doped with phosphoric acid as a material used to form a non-hydrated electrolyte membrane is disclosed in U.S. Pat. No. 5,525,436.
In addition, in fuel cells using a perfluorocarbonsulfonic acid polymer membrane which operates at a low-temperature, an electrode having hydrophobicity by being mixed with water-repellent polytetrafluoroethylene (PTFE) is used in order to improve gas diffusion, since gas is blocked by water generated in a cathode (Japanese Patent Laid-Open Publication No. hei 05-283082).
Phosphoric acid fuel cells that operate at a high temperature (150-200° C.) use phosphoric acid in a liquid state as an electrolyte. However, a large amount of the liquid state phosphoric acid in an electrolyte inhibits gas diffusion within the electrode. Thus, an electrode catalyst layer that prevents pores of the electrode from being blocked by the phosphoric acid by mixing the electrode catalyst with polytetrafluoroethylene (PTFE) having water repellency has been used.
In fuel cells applying polybenzimidazole (PBI) retaining phosphoric acid, which is a high-temperature non-hydrated electrolyte, to an electrolyte membrane, attempts to impregnate an electrode with a liquid state phosphoric acid have been made and attempts to increase the loading amount of a metal catalyst have been made in order to facilitate interface contact between the electrode and the membrane. These attempts, however, did not sufficiently improve characteristics of the fuel cells.
When air is supplied to a cathode, aging takes about a week even if an electrode composition is optimized in a solid polymer electrolyte membrane doped with phosphoric acid. Although a fuel cell can have improved efficiency and aging time can be decreased by replacing air with oxygen, use of oxygen is not desirable for commercialization. Furthermore, an electrolyte membrane prepared using a homopolymer of PBI does not have sufficient mechanical properties, chemical stability, and capacity of phosphoric acid at a high temperature.